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Home , Bikini Atoll, Castle Bravo, Ivy Mike, Nuclear weapons testing

maps of ocean sediment from the crater

Emlyn W. Hughesa,b,1, Monica Rouco Molinaa, Maveric K. I. L. Abellaa, Ivana Nikolic-Hughesa,c, and Malvin A. Rudermanb,1

aK=1 Project, Center for Nuclear Studies, Columbia University, New York, NY 10027; bDepartment of Physics, Columbia University, New York, NY 10027; and cDepartment of Chemistry, Columbia University, New York, NY 10027

Contributed by Malvin A. Ruderman, May 14, 2019 (sent for review , 2019; reviewed by David Kawall and Yury G. Kolomensky) On March 1, 1954, the conducted its largest comes from a complicated mix of nuclear weapons tests detonated test in Atoll in the ; in the area. The range of yields was from 1 to 15 megatons for the the detonation was code-named “Castle Bravo.” Radioactive de- 7 nuclear weapons detonations in the Bravo crater vicinity (Table posits in the ocean sediment at the bomb crater are widespread 1 and Fig. 1). The total yield from nuclear-weapons tests per- and high levels of contamination remain today. One hundred formed in the few-kilometer-diameter Bravo crater region corre- thirty cores were collected from the top 25 cm of surface sediment sponds to one-third of the total yield from the entire US nuclear 2 at ocean depths approaching 60 m over a ∼2-km area, allowing for weapons testing program in the Marshall Islands. a presentation of radiation maps of the Bravo crater site. Radio- Over the years, sediment samples have been collected in Bikini chemical analyses were performed on the following radionuclides: Atoll to determine the radioactive contamination of the lagoon -(239,240), plutonium-238, -241, bismuth-207, from the program. In the 1960s and and cesium-137. Large values of plutonium-(239,240), americium- 1970s, a handful of cores was collected in the Bravo crater and 241, and bismuth-207 are found. Comparisons are made to core the radionuclide presence was measured in these samples (5–7). sample results from other areas in the northern Marshall Islands. A recent study of ocean cores collected at the Bikini and Enewetak test sites provides an up-to-date quantitative result of Bravo crater | Bikini Island | cesium-137 | ocean sediment | plutonium radiation levels in the crater sediment (8). However, the study was limited to a few cores. No large-scale systematic study has n March 1, 1954, the US military detonated its largest been published on the radioactive contamination to ocean sed- Othermonuclear weapon on an island located in the north- iment from nuclear weapons testing. western rim of in the Marshall Islands. The weapon, Here, we present measurements of the radioactivity levels of 5 code-named Castle Bravo, released an energy equivalent to 15 radionuclides from 129 cores collected within the Bravo crater. million tons of trinitrotoluene (TNT) (15 megatons), a value These results are used to create radiation maps of the Bravo substantially larger than the US military’s prior estimates, al- crater, allowing for a more detailed and systematic discussion of though controversy still remains on this topic (1). [For example, radioactive contamination from thermonuclear weapons testing. declassified documents show that the routing and placement of observing aircraft were set for a 15-megaton (1, 2).] Results The Bravo bomb was the first thermonuclear weapon test using On August 1, 2018, we surveyed the depth of the Bravo crater solid LiD material as the central design for the fusion process, using a JMC V108 video echo sonar installed underneath the allowing this device to be aviation deliverable to enemy targets. The test was performed on a strip of land adjacent to Nam Island. The Bravo bomb was the second large-scale thermonu- Significance clear weapon test performed, following the first test, code named , whose yield was 10.4 megatons. Ivy Mike was deto- High-yield thermonuclear cause enormous radioactive nated in the northwest rim of 16 mo earlier, on contamination to the environment. These “hydrogen bombs,” , 1952. when tested on small islands in the ocean, vaporize the land and The Bravo bomb vaporized an artificial island, leaving a ∼75- produce radionuclides that settle in the ocean sediment. Even m-deep, 1.5-km-diameter crater located adjacent to a ∼0.25-km2 decades later, significant contamination may remain in the sedi- patch of land, called Nam Island (Fig. 1). Instantaneously, ment surface and deep into the sediment layers. Measuring the the Bravo test produced a blast, heat, and prompt radiation, which radioactive contamination of the crater sediment is a first step in dissipated rapidly (3). However, residual radiation from radioactive assessing the overall impact of nuclear weapons testing on the fallout (4) was spread throughout the world and, not surprisingly, a ocean ecosystems. We find radiation levels orders of magnitude particularly large concentration was deposited locally in the Bravo above background for plutonium-(239,240), americium-241, and crater in the northwest corner of the Bikini lagoon and throughout bismuth-207 in the top 25 cm of sediment across the entire Bravo the northern Marshall Islands. Residual radiation from such a bomb crater, the location of the largest aboveground US nuclear massive fallout can last from days to years, decades, centuries, and weapons test. beyond, depending on the half-life of the radionuclide. Nuclear weapons testing at the location of the Bravo crater did Author contributions: E.W.H., I.N.-H., and M.A.R. designed research; E.W.H. and M.R.M. performed research; E.W.H. and M.K.I.L.A. contributed new reagents/analytic tools; E.W.H., not end with the Bravo test. Six more nuclear weapons were M.K.I.L.A., and M.R.M. analyzed data; and E.W.H., I.N.-H., and M.A.R. wrote the paper. detonated in close proximity to the Bravo crater. Two tests in Reviewers: D.K., University of Massachusetts Amherst; and Y.G.K., University of particular, named Castle Romeo (detonated 1 mo after Bravo) California, Berkeley. and Hardtack Poplar (detonated in 1958), had yields of ∼10 The authors declare no conflict of interest. megatons each, and both were located southwest and approxi- This open access article is distributed under Creative Commons Attribution-NonCommercial- mately 2 km away from the Bravo center. These tests likely NoDerivatives License 4.0 (CC BY-NC-ND). knocked the southwestern rim of the initial Bravo crater, making 1To whom correspondence may be addressed. Email: [email protected] or mar@astro. the southern edge of the Bravo crater the deepest side. As a columbia.edu. result, today’s radioactive contamination from the Bravo crater Published online July 15, 2019.

15420–15424 | PNAS | July 30, 2019 | vol. 116 | no. 31 www.pnas.org/cgi/doi/10.1073/pnas.1903478116 Downloaded by guest on September 27, 2021 Fig. 2. Interpolated depth map of the Bravo crater based on echo sonar measurements (black dots).

Fig. 1. Approximate location of the 7 tests conducted by the United States average values of 56, 34, and 10 pCi/g, respectively. The un- inside the Bravo crater. GPS coordinates collected from ref. 16. certainty in the measurements was less than 1 pCi/g in all cases.

Before the primary radiation mapping project performed in SCIENCES

Indies Surveyor August 2018, we collected 21 cores from 1 location at the Bravo ENVIRONMENTAL hull of a 23-m research vessel, , owned by the center in May 2017. The main goal of the 2017 coring pilot study Indies Trader company. Fig. 2 presents the results of an inter- was to investigate the reproducibility of the ocean sediment re- polated depth map of the crater by performing a kriging fit to the sults, when collected from precisely the same location. Table 2 sonar data using the R programming language (9). The fit used a presents the results for the mean and SD from the 5 radionu- Gaussian model (10) similar to an analysis done on island gamma clides studied for these 21 cores. The analysis of the cores by radiation measurements (11). A map generated from individual Gel Laboratories is identical to what was performed on the depth measurements, collected by the scuba divers, allowed for an 2018 cores. In fact, all cores were analyzed in the same batch independent assessment of the accuracy of the depth measure- submission in Fall 2018. In Table 2, we also list the radionu- ments. From this comparison, we estimate an overall uncertainty clide half-life, which has an effect on the current levels for a ± on our depth map to be 1.5 m. The main reason for the depth few of the radionuclides, given that the weapons testing occurred study was to aid in the dive planning. Sediment collection in the 65 y ago. majority of the crater necessitated deep diving to depths greater From August 2 to 9, 2018, 108 sediment cores were collected than 45 m, firmly in the range of advanced technical scuba diving. along 4 diameters across the crater, corresponding to the north– A total of 129 cores was collected in the Bravo crater over 2 y, south, east–west, northwest–southeast, and northeast–southwest in 2017 and 2018. Measurements were made of the following ra- directions. Fig. 3 presents a map of the designed 100-core plan 238 239,240 dionuclides: plutonium-238 ( Pu), plutonium-(239,240) ( Pu), versus the actual Global Positioning System (GPS)-measured 241 207 americium-241 ( Am), bismuth-207 ( Bi), and cesium-137 coordinates of the 108 cores that were collected. The origin of (137Cs). Relatively low values were found for 2 radionuclides, the difference between our goals and the true locations come namely 238Pu and 137Cs. These radionuclides were chosen for our from the limitation in the scuba diver’s ability to estimate the study as they represent by-products from nuclear weapons testing distance and direction traveled deep underwater using a compass and were accessible for Gel Laboratories, the company that per- and a dive propulsion vehicle (DPV). formed the radiochemical analysis. The average values for 238Pu For the 108 cores collected in 2018, the individual radiation , and 137Cs were found to be 2.3 and 1.6 pCi/g, respectively. The maps for the Bravo crater for 238Pu, 239 240Pu, 241Am, 207Bi, and , values for 239 240Pu, 241Am, and 207Bi were noticeably higher with 137Cs are given in Fig. 4. All results come from performing a kriging fit to the raw data. In general, one sees a relatively flat

Table 1. Nuclear weapons test in the Bravo crater region Table 2. Mean and SD activity levels from radionuclides in 21 Name Date Yield, kilotons sediment cores collected at the same location, near the center of Bravo 3/1/54 15,000 the Bravo crater Romeo 3/27/54 11,000 Radionuclide Mean SD Half-life, y Fir 5/12/58 1,360 Sycamore 6/1/58 92 Plutonium-239,240 50 6 24,000, 6,560 Aspen 6/15/58 319 Americium-241 34 8 432 Cedar 7/3/58 220 Plutonium-238 2.3 0.6 88 Poplar 7/13/58 9,300 Bismuth-207 13 7 32 Total 37,391 Cesium-137 1.7 0.2 30

The columns list the test name, the date of the test, and the yield in Results are given in units of pCi/gm. Half-lives of the radionuclides are also kilotons. Total yield in the Bravo crater region is also presented. presented.

Hughes et al. PNAS | July 30, 2019 | vol. 116 | no. 31 | 15421 Downloaded by guest on September 27, 2021 , Rongelap Island in , and several cores from the Lacrosse crater next to in Enewetak Atoll. Table 3 presents single-core results for the 5 radionuclides collected from these regions compared with the average results from the Bravo crater. Not surprisingly, contamination levels farther away from the nuclear weapons testing sites have reduced radiation levels. Discussion The core samples from the Bravo crater provide radiation data of the lagoon sediment with sufficient statistics to produce interpolated maps. This study allows for the possibility to observe, for example, potential general trends and hot spots in the measured radionu- clide distributions. Overall, we find a fairly uniform distribution of , 239 240Pu throughout the crater, with an observable tapering off toward the rim of the crater. A notably sharper tapering is observed in the 207Bi results, from high values west of center to nearly un- detectable levels at the crater edges. The 137Cs data display a de- crease by the northeastern crater edge; however, the overall levels of 137Cs are substantially lower on average than all but the 238Pu , results. The 241Am data correlate quite strongly with the 239 240Pu data; this is a topic for future study. The presence of coral, alive and dead, and notably grainy sand at the crater rim as well as the washout from weather events in Fig. 3. Desired location (white circles) versus actual location (red circles) of shallower regions could affect and decrease the apparent activity. 137 collected sediment cores in the Bravo crater. Some radionuclides, such as Cs, are well known to interact chemically with the environment. Compared with other regions in the northern Marshall Islands distribution with larger values located more centrally in the (Table 3), we find the largest extent of radionuclide activity to be crater followed by tapering off by the crater edges, with some in the Bravo crater. Compared with the lagoon near Bikini and notable exceptions. Enyu Islands on the eastern side of Bikini Atoll, the values are an Finally, a comparison is made to cores collected from other order of magnitude higher. In Utirik Atoll, ∼500 km away, we islands and atolls in the northern Marshall Islands; we collected find no presence of residual radioactivity to within the precision cores from additional locations just outside of islands, namely of our measurement uncertainty. Interestingly, there is also a near Bikini and Enyu Islands in Bikini Atoll, Utirik Island in significant difference between the lagoon near Bikini versus

Fig. 4. Radiation levels of 5 different radionuclides in the top 25 cm of surface sediment of the Bravo crater. (A) 238Pu. (B) 239/240Pu. (C) 137Cs. (D) 241Am. (E) 207Bi. All activity concentrations are given in units of picocuries per gram.

15422 | www.pnas.org/cgi/doi/10.1073/pnas.1903478116 Hughes et al. Downloaded by guest on September 27, 2021 Table 3. Activity levels for 5 radionuclides collected from difficult challenge. Follow-up studies of the thermonuclear bomb individual cores adjacent to islands in the northern Marshall crater environments would best be conducted in shallower con- Islands ditions. For example, working in the Ivy Mike crater, whose Radionuclide Bravo Bikini Enyu Rongelap Utirik Medren Runit depth is on the order of 35 m, as shown in Fig. 5, offers nu- merous advantages. Although still formally within the depths Plutonium-239,240 54 2.3 0.45 0.23 0 0.17 6.2 defined by technical scuba diving, the requirements on the diver Americium-241 40 1.5 0.4 0 0 0.2 1.0 would be eased substantially. Similarly, fission bomb craters, Plutonium-238 2.3 0.2 0 0 0 0 2.0 such as those found near Runit Island in Enewetak Atoll, are Bismuth-207 15 0 0 0 0 0 0 shallow and may be easier to study from a data collection Cesium-137 1.6 0 0 0 0 0 0.6 perspective. In summary, there is still residual contamination of radionu- The Bravo results come from an average over 108 cores. Results are given in pCi/gm. clides throughout the Bravo bomb crater, from center to rim. We find that the radionuclide distribution is fairly uniform across the crater with some tapering off toward the crater rim, the most 207 Enyu Island, with Enyu displaying noticeably smaller values for dramatic change being in Bi. Although the lagoon is gradually , 239 240Pu, for example. Although we did not collect core samples filling in over time, contamination levels from residual long-lived from other thermonuclear bomb craters, we did collect a few radioactive isotopes, such as plutonium and americium, will likely samples from a fission bomb crater neighboring Runit Island in last for centuries. The tests caused a dramatic Enewetak Atoll. The Runit crater has an order of magnitude less change in sediment composition. Additional studies to determine , 239 240Pu contamination, but interestingly still has a concentra- what the impact on life is in the lagoon craters, especially at the tion of 238Pu comparable to the Bravo crater. deeper depths, would be valuable. Compared with the recent handful of cores collected by the Methods Buesseler et al. (8), our average results are in good agreement. We find an average value of 56 picocuries per gram (pCi/gm) for Crater Depth Mapping. To plan the dive program, the first step was to map the 239,240 crater. The 23-m Indies Trader Indies Surveyor ship was equipped with a JMC Pu activities in the Bravo crater, relative to 49 pCi/g (1,820 V108 video echo sonar installed underneath its hull. The sonar determined Bq/kg) from the top sediment of craters noted in the Buesseler the lagoon depth over a series of evenly spaced positions. From these et al. study. measurements, we generated a depth map using a kriging fit implemented SCIENCES

There are uncertainties in the results from the present study, with the R programming language (9). Mapping of the Bravo and Mike ENVIRONMENTAL some pertaining to the cores themselves and some related to the craters, using the Surveyor, took several hours each. chemical analysis. The sediment was collected down to 25-cm depth from its surface. For the majority of the cores, the sedi- Sediment Core Collection. Sediment cores were collected in 5-cm-diameter, ment analyzed corresponded to depths between ∼15 and 25 cm 25-cm-long cylindrical clear plastic PVC tubes. The tubes had machined deep. However, some of the cores, especially those near the teeth on 1 edge, which were screwed into the top layer of the lagoon sediment during the collection process. Once the cores were filled, a vacuum- Bravo rim, were not packed as tightly, so there could have been tight seal was screwed onto the exposed end of the sediment core. The core mixing between different sediment depths. It is likely that the was then lifted and retrieved from the sediment floor and immediately average true sampled sediment depth coming from cores col- capped. The task was performed at depths down to ∼60 m in low-visibility lected near the Bravo crater rim is shallower than the depth from conditions. Collecting the sediment disturbed the lagoon floor and rapidly cores collected near the Bravo crater center. A more systematic generated a large silt cloud over most of the Bravo crater. Minimal sediment comparison of cores in which the core depth information is kept was lost during the retrieval process due to the strong suction from the first strictly intact would be valuable in future studies, as has been seal. Once the cores were capped, they were immediately sent to the surface using lift bags. done previously (8, 12). Once a sediment core was collected by a pair of divers, the dive team The crater appears to be filling in at a rate of approximately a navigated underwater to a neighboring point, typically 60 m away, using a meter every 3 y, so the measurement of the radioactive con- compass and a single Minnus-Traveler DPV made by the Submerge Scooters tamination performed today is only a crude approximation to what it was at the time of the Bravo test, 65 y ago. Contamination from neighboring nuclear tests, such as Romeo and Poplar, also cannot be excluded as contributing to the overall contamination in the Bravo crater lagoon sediment. The existence of these additional tests underscores the complexities in terms of esti- mating lagoon sediment contamination from individual nuclear weapons tests. The properties of the individual weapon tests also play a large role in the ultimate radionuclide deposition. In regard to studies of nuclear weapon bomb craters in ocean environments, an important issue is the recovery of, in this case, lagoon life. Some studies addressing these issues have been done in Bikini Atoll (13, 14). On the edges of the crater at shallow depths, this pertains primarily to the coral regrowth. At deep depths, the sediment composition from nuclear weapons testing is dramatically altered, and a much finer grain and silty sediment remains. Bacteria and sea cucumbers are ubiquitous. Studies of the morphology have been performed (15). Correlating residual radioactivity and sediment conditions with lagoon life impact would be an interesting future study. Studying the Bravo crater and collecting ocean sediment was a large undertaking that involved deep technical scuba diving and the use of advanced scuba equipment and technologies. The Fig. 5. Interpolated depth map of the Ivy Mike crater based on echo sonar zero-visibility conditions at deep depths presented a particularly measurements (black dots).

Hughes et al. PNAS | July 30, 2019 | vol. 116 | no. 31 | 15423 Downloaded by guest on September 27, 2021 Company. The DPV used a NiMH battery so that it could be transported on a than closer to the surface. However, the sediment composition over the commercial aircraft. The scooter was able to rapidly transport a pair of divers majority of the Bravo crater did differ significantly from the composition near at depth. Five advanced technical divers performed the crater sediment the edges. Samples collected near the crater edges were more grainy; collection for the radiation mapping project over an 8-d period. therefore, these samples are more likely to have sediment mixing from differing sediment depths (ranging, at most, over the full 0 to 25 cm). GPS Measurements. A GPS record was taken on the surface by hand using a Garmin etrex 30x by researchers operating on a small motor boat. Multiple Radiochemical Analysis. Gel Laboratories performed the radiochemical analysis GPS measurements were taken by the team on the boat for redundancy. The of the core sediment samples. The subsample precipitate was dissolved in acid, Garmin GPS measurement is quoted by the company as being accurate to a and the elements were subsequently separated by ion exchange resins. After distance of ±12 m. A dedicated study was performed in which multiple cores radiochemical separation, concentrations were determined by isotopic alpha were sent from the same location at deep depths in rapid succession, and emission (16). Minimum detection limits for alpha emission are estimated to be the GPS point was measured on the surface, under windy conditions. From 1 pCi/g. Measurements of 137 Cs activity were performed using gamma de- this systematic study, we found a reproducibility of the core location to be good to within a couple of meters. The relative precision of the core GPS tection techniques, and minimum detection limits for this process are esti- measurement appears to be significantly better than the absolute position mated to be 0.1 pCi/g. uncertainty quoted by the Garmin Company. Although on a few windy days the surface of the water in the lagoon became quite choppy, there was ACKNOWLEDGMENTS. We thank the people of Bikini for supporting this essentially no current in the Bravo crater throughout the project. project, and the Marshall Islands Nuclear Commission for guiding us through the approval process. We thank Indies Trader for providing the infrastruc- ture, transportation, and overall support for the dive project. We are in- Subsample Preparation. Once collected, cores were transported intact back to debted to Tamara Adame, Mara-Teodora Feodor, Cyrielle Houard, Brian Columbia University. Subsamples of sediment were removed from the deep Kirk, Camille Laquerriere, and Leigh Tobin for their heroic effort in collecting end of the core, corresponding to 15 and 25 cm from the lagoon sediment the sediment samples in the Bravo crater. The divers performed a complicated surface. Subsamples were transferred to a 100-mL plastic centrifuge tube. All task-loading activity, while deep technical scuba diving in zero visibility condi- subsamples were mailed as a batch to Gel Laboratories for isotopic analysis. tions. We thank Submerge Scooters for their support and production of the Although sediment mixing throughout the 25-cm depth is certainly possible, dive propulsion vehicles that were critical to this project, operating in a partic- it did appear that for the vast majority of the core samples the solid hard mud, ularly isolated and remote location. We thank Autumn Bordner and Gemma with pulverized consistency of the lagoon sediment, is more likely not to have Sahwell for aiding in the GPS data collection. We also thank Gemma Sahwell for mixed. Therefore, it is likely more accurate to assume that the samples in this collecting cores from regions outside the Bravo crater. We thank Gel Laborato- study are represented by considering that the composition originated from ries for their speedy delivery and professionalism pertaining to the isotope sediment located between 15 and 25 cm from the sediment surface rather analysis results. This project was funded and supported by Columbia University.

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15424 | www.pnas.org/cgi/doi/10.1073/pnas.1903478116 Hughes et al. Downloaded by guest on September 27, 2021